For example, when measuring the amount of particulate matter (hereinafter also referred to as PM) having a predetermined particle size contained in exhaust gas, the exhaust gas is diluted at a large dilution ratio, and the diluted exhaust gas is passed through a filter at a predetermined flow rate.
As a dilution ratio is increased, an error in flow rate of dilution air flowing into a diluter, and/or an error in flow rate of exhaust gas got by the diluter become more likely to affect the accuracy of the dilution ratio. For this reason, as a dilution ratio is increased, it becomes more difficult for an exhaust gas sampling apparatus including only a single-stage dilution flow path to dilute exhaust gas with a predetermined accuracy.
In order to solve such a problem, as disclosed in Japanese Patent No. 3054514, there has been proposed an exhaust gas sampling apparatus including a two-stage dilution flow path. This exhaust gas sampling apparatus is configured to dilute exhaust gas at a dilution ratio R1 in a first stage dilution flow path, and in a second stage dilution flow path, get a part of the diluted exhaust gas resulting from the dilution in the first flow path to further dilute it at a dilution ratio R2. Each of the dilution flow paths is provided with: a dilution air flow rate controller adapted to control the flow rate of inflow dilution air; and a diluted exhaust gas flow rate controller adapted to control the flow rate of outflow diluted exhaust gas. That is, the exhaust gas sampling apparatus dilutes the exhaust gas at a desired dilution ratio R by controlling the four flow rates using the four flow rate controllers in total, respectively.
However, in the exhaust gas sampling apparatus described in Japanese Patent No. 3054514, when changing a final dilution ratio R, the flow rates to be controlled by all the four flow rate controllers have been changed so as to accurately dilute the exhaust gas. In addition, in order to respond to various dilution ratios, it has been considered that variable flow rate ranges of the respective flow rate controllers must be set to be large, thus causing increases in size and complexity of a system.
Further, as a result of intensive examination by the present inventor, the inventor has first found that an exhaust gas sampling apparatus of a multistage dilution type can make the accuracy of a final stage dilution ratio higher than before while simplifying a system configuration.